1. Field of the Invention
The invention relates to co-curable elastomer compositions based on a blend of halogenated isomonoolefin/para-alkylstyrene copolymers and olefinically unsaturated elastomers.
2. Description of Related Art
Vulcanizates based on blends of elastomers which contain little or no olefinic unsaturation with more highly unsaturated elastomers are of interest in the rubber industry primarily because of their special properties, e.g., superior resistance to ozone degradation and consequent cracking, improved resistance to chemical attack, improved temperature stability and unique dynamic response. These blends can permit the achieving of synergisms wherein the composite blend possesses combinations of properties unattainable in the individual elastomers. However, these desirable properties can be realized only when an intimate homogeneous blend of the elastomers with phase sizes of less than 5 microns, generally 1 micron or less, is produced and maintained in the blend and a satisfactory level of interfacial adhesion is achieved.
Unfortunately, it generally known that most polymers are not compatible with one another unless specific favorable interactions are present because the favorable entropy of mixing is too small to overcome the unfavorable enthalpy of mixing such that the free energy of mixing is unfavorable. Blends produced by normal techniques are grossly inhomogeneous with phase sizes many microns in diameter. This gross incompatibility of the individual polymers with a consequent inability to produce and maintain the homogeneous fine phase sizes required in synergistic blends is particularly evidenced when the individual polymers differ considerably in solubility parameters as is the case when attempts are made to blend low unsaturation elastomers with the more highly unsaturated elastomers. A further problem is that, even if intimate dispersions can be produced during high shear mixing operations, the mixtures phase-separate when the mixing is stopped so that the final blends are grossly inhomogeneous with the individual phase sizes many microns in diameter. These grossly inhomogeneous blends generally have very poor combinations of properties, usually much worse than the individual polymers, rather than displaying the desirable synergistic combination of properties obtainable in the more intimate homogeneous blends with phase sizes less than 5 microns, preferably 1 micron or less.
Still other problems with blends of low unsaturation elastomers with the more highly unsaturated elastomers are the problems associated with the curing of elastomer compositions containing such chemically diverse materials and the inability to achieve a balanced cure of each individual component and also a truly intervulcanized composition, i.e., a composition where predominant interpolymer crosslinking takes place between different polymer molecules in the different phases. For example, in sulfur curable systems containing a blend of highly unsaturated elastomers such as natural rubber and low unsaturation elastomers such as butyl rubber, the high unsaturation diene phase is much faster curing than the low unsaturation phase and the curatives rapidly diffuse out of the low unsaturation elastomer phases into the high unsaturation elastomer phases resulting in a highly cured polydiene phase and an undercured, curative-starved low unsaturation phase, with little or no interpolymer crosslinking taking place at the phase boundaries. As a consequence of this lack of curing balance, interfacial adhesion is low and the phase sizes are large, and the vulcanizates exhibit inferior mechanical properties such as low tensile strength, low modulus, poor hysteresis and the like.
One technique used to minimize the problem of vulcanization imbalance in such blends is the use of low or no unsaturation blend components which have been modified by the inclusion of functional groups, e.g. halogen, in the polymer chain which functional groups are susceptible to crosslinking mechanisms independent of the sulfur curing system used to crosslink the highly unsaturated elastomer. For example, blends containing halogenated interpolymers of isobutylene and para-methylstryene can be vulcanized along with more highly unsaturated rubbers by including an independent curing system for each type of elastomer into the vulcanization recipe, e.g., a zinc oxide-based curing system which normally cures the halorubber and an accelerated sulfur curing system which normally cures the highly unsaturated rubber. This technique can overcome the cure imbalance but doesn't affect the gross incompatibility discussed above, doesn't produce high interfacial adhesion and does not give rise to truly intervulcanized blends.
Examples of such blends are those containing: (a) one or a mixture of low unsaturated rubbers such as halogenated interpolymers of a C.sub.4 to C.sub.7 isomonoolefin, e.g., isobutylene, with up to about 20 wt % para-alkylstyrene, e.g., para-methylstyrene, mixed with: (b) one or more olefinically unsaturated elastomers such as natural rubber, polybutadiene, polyisoprene, copolymers of butadiene with styrene or acrylonitrile, and the like.
An Exxon Chemical company brochure entitled "Exxon Bromo XP-50 Rubber Compounding and Applications," May, 1992 discloses that brominated isobutylene/para-methylstyrene copolymers may be self-cured using a combination of zinc oxide and stearic acid, used at levels of about 1.0 and 2.0 parts by weight respectively per 100 parts of rubber (phr). The reference further teaches that neither zinc oxide or stearic acid used alone will cure the rubber, but zinc stearate used alone at levels above 3.0 phr will give rise to fast, scorchy cures which tend to revert on aging. At lower levels, zinc stearate used alone gives slow and impractical cure rates.
In addition, a series of papers presented by Exxon Chemical Company at the Spring and Fall 1992 meetings of the Rubber Division of the American Chemical society suggest that zinc bromide and combinations of zinc bromide or zinc oxide with stearic acid, are effective to varying degrees not only for the self-cure of brominated isobutylene/para-methylstyrene copolymers (hereafter referred to as BI-PMS) but also for the co-cure of such materials when mixed with more highly unsaturated elastomers such as natural rubber, polybutadiene and similar materials. It was concluded in these studies that the curing reactions give rise to both intramolecular curing, i.e., electrophilic substitution reactions involving aromatic moieties in different BI-PMS molecules, as well as interpolymer curing involving electrophilic addition reactions of benzylic bromine across double bonds present in the unsaturated elastomer.
In all these co-cure systems, two competing reaction mechanisms (self-curing and co-curing) are occurring to varying degrees which means that exclusive interpolymer curing between the BI-PMS molecular chains and the unsaturated elastomer chains is not achieved. This curing imbalance may reflect itself as a drop-off of important properties such as modulus, tensile, elongation and the like in comparison to what would be expected as the composition-based averaging of such properties based on the properties exhibited by each cured elastomer alone, i.e., the "tie line" properties.
It should be noted that the achievement of tie line or above tie line rheometer cure performance in curable elastomer blend systems is an unusual phenomenon and represents optimum cure performance. In most systems, rheometer torque increase for cured blends of different elastomers will lie at least partially below the tie line which would be graphically depicted as a straight line over the blend range connecting the rheometer torque increase values associated with each individual elastomer if cured alone. One blend system which allegedly achieves above tie line performance is disclosed in Hopper et al., "Ozone Resistant Blends", International Conference on Advances in the Stabilization and Controlled Degradation of Polymers, Lucene, Switzerland, May 23-25, 1984. The publication discloses sulfur curable blends of modified EPDM rubber and a polydiene rubber such as natural or polybutadiene rubber exhibiting blend torque increases which are generally slightly above tie line values. The modified EPDM employed is the addition product of an N-chlorothio-N-methyl-p-toluene sulfonamide to elastomeric terpolymers of ethylene, propylene and a non-conjugated diene such as 1,4-hexadiene or dicyclopentadiene.